For load balanced clusters, we have to include statistics about the number of connections being pinned by cursors and transactions in wait queue timeout errors:

The driver connection pool MUST track the purpose for which connections are checked out in the following 3 categories:

• Connections checked out for cursors
• Connections checked out for transactions
• Connections checked out for operations not falling under the previous 2 categories

When the connection pool's maxPoolSize is reached and the pool times out waiting for a new connection the WaitQueueTimeoutError MUST include a new detailed message, "Timeout waiting for connection from the connection pool. maxPoolSize: n, connections in use by cursors: n, connections in use by transactions: n, connections in use by other operations: n".

https://github.com/mongodb/specifications/blob/master/source/load-balancers/load-balancers.rst#connection-tracking